Logging tools for measuring the formation density in open holes are well known. Currently-available "open-hole" density logging tools normally include a gamma ray source, typically cesium-137, and two detectors--a near detector and a far detector. Open-hole density tools have two detectors to compensate for mudcake (i.e. a layer of solid material consolidated from drilling fluid that normally lines an open borehole) and standoff (i.e. the distance between the tool and the side of the borehole). Gamma rays are continuously emitted from the source and propagate out through the mudcake and into the formation. The electron density of the formation is calculated based on the count rate or intensity of the gamma rays that are received at the detectors after passing through the mudcake and formation.
In addition to employing radioactive sources for gamma rays, it is anticipated that density tools will employ accelerators, or more specifically electron accelerators, as gamma ray sources even though such accelerators are not currently available. In tools employing accelerators, the detectors will not necessarily detect and/or calculate gamma ray count rates because most accelerator designs generate relatively huge amounts of gamma rays for brief, intermittent periods of time resulting in large quantities of gamma rays engaging the detectors within a relatively short period of time. Therefore, it may be more feasible to detect and measure the intensities of the gamma rays rather than the count rate or the rate at which gamma rays engage the detectors. The detectors would generate voltage signals proportional to the intensities of the detected gamma rays as opposed to count rate signals.
In general, a layer of mudcake exists along the outer periphery of the borehole. The intensity signals from the near and far detectors are combined to provide a measure of the formation density that is essentially independent of this mudcake if the thickness of the mudcake is within a limited range. This is possible when the spacings and collimations chosen for the two detectors result in substantially different sensitivities to the mudcake and formation. The thickness of the mudcake that can be compensated for is limited by the spacing between the near detector and the gamma ray source and by the collimation of the near detector. A longer spacing between the gamma ray source and the near detector enables more mudcake to be penetrated by the gamma rays and therefore enables the tool to compensate for thicker mudcake. However, greater spacings between the gamma ray source and the near detector reduce the contrast between the near and far detector sensitivities to mudcake and the formation, thereby reducing the quality of the density measurement.
However, these two-detector density logging tools are limited in their use in that they are unable to provide an accurate formation density when used in a cased hole. Specifically, while the currently-available two detector density tools can compensate for mudcake, they are unable to compensate for the casing and cement and are therefore unable to accurately calculate the formation density in a cased hole.
The gamma ray detectors consist of sodium iodide crystals connected to photomultiplier tubes. These devices are commercially available and well known in the art. When gamma rays interact in the sodium iodide crystal, they produce light which is converted by the photomultiplier tube into an electronic signal. These signals are amplified to produce voltage pulses that are proportional to the energy deposited in the crystal. These voltage pulses may be counted to determine the gross number of gamma rays detected per second or they may be sorted by amplitude to determine the number of gamma rays detected per second in various energy ranges. The later technique is common in open-hole logging, as described in the paper "A New Approach to Determining Compensated Density and P.sub.e Values with a Spectra-Density Tool", presented by G. L. Moake at the Annual SPWLA Logging Symposium in Midland, Jun. 16-19, 1991.
Typical two detector density tools are decentralized with a bowspring or caliper device so that the detectors and the source are pushed up against the side of the borehole. The decentralization of the tool against the side of the borehole substantially precludes the gamma rays from propagating upward through the drilling fluid and therefore the effect of the drilling fluid on the density measurement is minimized.
However, as noted above, the two detector density tools cannot adequately function in cased holes because they are unable to compensate for the casing and the cement. In short, too many variables are present in order to accurately calculate the formation density. Specifically, in order to probe the formation, the gamma rays must exit the tool, pass through the casing and cement and scatter in the formation before passing back through the cement and the casing to finally reenter the tool to be detected. Thus, instead of just a mudcake correction as in the case of open holes, a cased hole density tool must be able to correct or compensate for the cement and casing which are both more formidable a correction than a layer of mudcake because of the relatively high densities of metal casing and thick amounts of cement (i.e. fewer gamma rays are able to penetrate the casing and cement to reach the near detector).
Occasionally, open hole density tools are used to run cased hole density logs. The information provided by open hole density tools in cased holes is accurate in only a limited number of cases, primarily when the cement thickness is quite small or no cement is present. This can occur on the low side of deviated wells where the casing may very well be resting against bare formation or only a small amount of cement will be disposed behind a casing. Typically, open hole density tools employ a gamma source in the form of contained cesium-137 which produces 0.662 MeV gamma rays. The amount of cement that can be corrected for is limited by the near or "first" detector. Typically, these near detectors can correct for only about 0.75 inches of mudcake in an open hole and therefore it is easy to see why these tools are ineffective in compensating for steel casing, which is normally 0.36 to 0.50 inches thick in combination with a layer of cement disposed behind the casing. In any event, even dangerously thin casing is much denser, and therefore more effective at blocking the passage of gamma rays, than mudcake.
As the gamma rays pass through the casing, cement and formation, they are scattered and absorbed by these materials. The primary scattering interaction is called Compton scattering and is caused by electrons in the casing, cement and formation. Compton scattering tends to reduce the energy of the gamma rays that are scattered until their energy is so low that the gamma rays are absorbed by electrons through an interaction called photoelectric absorption. Thus, if the density of electrons in the casing, cement and formation is high, fewer electrons will make it back through the formation, cement and casing to be detected in the tool. In short, the number of detected gamma rays varies inversely to the density of electrons in the various materials through which the gamma rays must pass. Since the density of electrons is directly related to the density of the material being measured, the number of detected gamma rays will also depend strongly on the density of the casing, the density of the cement and the density of the formation.
Two other physical processes that dominate the scattering and absorption of gamma rays are known as coherent scattering and photoelectric absorption, both of which are highly dependent upon the composition of the material. Those skilled in the art commonly represent the strength or effect of the coherent scattering and photoelectric absorption interactions relative to the strength of Compton scattering with the variable P.sub.e. Thus, the amount of gamma rays detected at the tool will also depend on the P.sub.e of the casing, the P.sub.e of the cement and the P.sub.e of the formation. Of course, the detected gamma rays will also depend on the thickness of the casing and the thickness of the cement.
As stated above, the performance of the tool will also depend slightly upon the density of the borehole fluid. However, density tools are commonly decentralized in the borehole, meaning that the tool, including the source and detectors is pushed up against the side of the borehole. In that manner, there is very little borehole fluid through which the gamma rays must pass in order to be detected at the tool. Further, since the borehole fluid properties are usually known and in any event can be tested at the surface, small correction factors based on the knowledge of the borehole fluid properties can be easily utilized.
Accordingly, there are eight variables that affect the response of the tool: the density of the casing (.rho..sub.s); the thickness of the casing (t.sub.s); the P.sub.e of the casing; the density of the cement (.rho..sub.c); the P.sub.e of the cement; the thickness of the cement (t.sub.c); the formation density (.rho..sub.f); and the P.sub.e of the formation. However, all but three independent combinations of variables can be accounted for prior to measurement.
First, casings tend to be made of the same material, namely carbon steel. Therefore, casings tend to have about the same density (.rho..sub.s) and P.sub.e. Thus the density and P.sub.e of the casing are known.
Second, the P.sub.e of most cements is a negligible value because the low energy gamma rays that are sensitive to the P.sub.e of the cement are blocked by photoelectric absorption that occurs within the casing. Accordingly, only the density and thickness of the cement are important. Further, the variations in the intensity signal of detected gamma rays at the tool due to the presence of cement behind the casing can be thought of as changes to the intensity signal that would be obtained where no cement is present at all. The cement can be thought of as a local substitution for formation. That is, it is reasonable to expect the intensity signal to vary with the difference between the formation density and the cement density as well as with the cement thickness. If either the contrast in the densities of formation and cement or the cement thickness is zero, then the effect of the cement is also zero. Therefore, the effect of the cement on the detected intensities at the tool will depend primarily on the following expression: EQU (.rho..sub.f -.rho..sub.c)t.sub.c
where .rho..sub.f is the formation density, .rho..sub.c is the cement density and t.sub.c is the cement thickness. The expression (.rho..sub.f -.rho..sub.c)t.sub.c can be treated as one variable. Similarly, the effect of the casing on the detected intensities at the tool will be proportional to the following expression which also can be treated as one variable: EQU (.rho..sub.f -.rho..sub.s)t.sub.s
where .rho..sub.s is the density of the casing, t.sub.s is the thickness of the casing and .rho..sub.f is the formation density.
Further, the P.sub.e of the formation can also be neglected because P.sub.e is a much lower value for high energy gamma rays than for low energy gamma rays. In other words, the effect of coherent scattering and photoelectric absorption is very low for high energy gamma rays; in contrast, the effect of coherent scattering and photoelectric absorption is relatively significant for low energy gamma rays. However, any gamma rays that become low energy gamma rays in the formation are unlikely to penetrate the cement and casing before being detected by the tool. Thus, the measured intensity at the tool will not depend significantly on the P.sub.e of the formation.
Therefore, in light of the above, there are only three independent variables that significantly affect the measured intensities: the density of the formation (.rho..sub.f); (.rho..sub.f -.rho..sub.c)t.sub.c ; and (.rho..sub.f -.rho..sub.s)t.sub.s. Because there are three unknown variables, three independent intensities must be measured in order to determine the formation density in a cased hole. Further, assuming a casing density (.rho..sub.s) and a cement density (.rho..sub.c), one can calculate the casing thickness (t.sub.s) and cement thickness (t.sub.c) respectively.
Accordingly, it would be highly desirable to provide a density logging tool with at least three detectors that would be capable of measuring the formation density in a cased hole as well as cement thickness and casing thickness. An effective cased hole density logging tool in combination with already existing cased hole neutron logging tools would provide an effective means for finding additional gas reserves behind existing cased holes. Further, an effective cased hole density tool would also provide important information regarding formation evaluation behind the casing. It is further contemplated that an improved cased hole density logging tool could also prove useful in open holes.
An improved tool for measuring formation density in cased or open holes is provided in the form of a housing that accommodates a gamma ray source and at least three detectors. The gamma ray source is spaced axially from the first, second and third detectors. The first or near detector is axially spaced from the gamma ray source by a distance defined as a first spacing. The first spacing and collimation for the first detector are designed so that the gamma rays detected at the first detector are those gamma rays that are scattered primarily by the casing. In short, the first detector primarily detects or preferentially detects gamma rays scattered through the casing.
A second or middle detector is spaced axially farther away from gamma ray source than the first detector. The second detector is spaced from the gamma ray source by a distance defined as a second spacing. The second spacing and collimation for the second detector are designed so that the gamma rays detected at the second detector will be those that are primarily scattered by the casing and the cement. Finally, a third or far detector is spaced axially farther away from the gamma ray source than both the first and second detectors by a distance defined as a third spacing. The third spacing and collimation defined by the third detector are designed so that the gamma rays detected at the third detector are those primarily scattered from the casing, cement and formation. It is this third detector that enables the tool to measure formation density while the first and second detectors primarily enable the tool to correct for casing and cement. However, the second detector can be used to measure formation density in the absence of cement.
Preferably, the detectors are shielded by a high density material that prevents detection of gamma rays that are simply travelling up through the tool. A pathway or void in the shielding is provided in the form of a collimation channel which extends from the detector through the tool and terminates at the outside surface of the tool. The collimation channels are specifically designed for the purpose of each detector. Specifically, the near or first detector will have a collimation that is aimed at a steep angle with respect to the casing so that the first detector will detect gamma rays that are scattered through all of the casing. The second or middle detector will have a collimation that is directed at a shallower or more perpendicular angle with respect to the casing because the second detector is intended to detect gamma rays scattered through all of the cement as well as the casing. Finally, the third or far detector will have a wide collimation channel which is directed substantially perpendicular to the casing due to the distance of the third detector from the source. Because gamma rays detected at the far detector must pass through the casing, cement, formation before passing back through the cement and casing, the statistical probability of this event happening is smaller than for the first and second detectors and therefore a wider collimation channel is required for the third detector.
It is also anticipated that a high energy gamma ray source such as cobalt 60 which provides 1.173 and 1.333 MeV gamma rays will be utilized. Other gamma ray sources, such as cesium-137 which provides only 0.662 MeV gamma rays or accelerator sources such as an electron accelerator could also be used. However, the spacing between the third or far detector and the gamma ray source will be longer than the conventional spacing between the gamma ray source and far detector in a open hole tool and therefore a source which emits higher energy gamma rays (i.e., 1.173 and 1.333 MeV) will provide a higher intensity at the far detector.
The present invention also lends itself to an improved method of measuring formation density behind casing as well as cement thickness and casing thickness. Specifically, the casing, cement and formation are irradiated with gamma rays. Gamma rays are detected at a first detector spaced a first axial distance from the gamma ray source. A first intensity is calculated for the first detector. Gamma rays are also detected at a second detector disposed a second axial distance from the gamma ray source. The second detector being disposed farther from the source than the first detector. A second intensity is calculated for the second detector. Gamma rays are also detected at a third detector spaced a third axial distance from the gamma ray source. The third detector is spaced farther from the source than both the first and second detectors. A third intensity is calculated for the third detector. An estimated formation density, cement thickness and casing thickness is then calculated based on the first, second and third intensity signals and by assuming constant values for the density and P.sub.e of the casing, negligible values for the P.sub.e of the cement and the P.sub.e of the formation and using a correction factor for the borehole fluid.
It is therefore an object of the present invention to provide an improved formation density tool for use in cased holes.
Another object of the present invention is to provide a logging tool which is capable of measuring formation density while compensating for the effects of casing and cement.
Another object of the present invention is to provide an improved three detector formation density tool.
Another object of the present invention is to provide an improved method for measuring formation density behind casing.
Another object of the present invention is to provide an improved gamma ray density tool capable of measuring cement thickness and casing thickness.
Yet another object of the present invention is to provide an improved formation density tool that may be used in both open and cased holes.
Other features and advances of the present invention will appear in the following description in which one embodiment has been set forth in detail in conjunction with the accompanying drawings.